linuxkpi: Add `timer64_to_tm()`

[dumbbell]

Summary

The function converts a number of seconds since Epoch to a struct tm.

The implementation was copied from contrib/tzcode/localtime.c.

The amdgpu DRM driver started to use it in Linux 6.15.

Sponsored by: The FreeBSD Foundation

[bz]

sys/compat/linuxkpi/common/src/linux_time.c

11 ↗

(On Diff #183793)

Which is public domain. What does that make this file then?

[dumbbell]

Fix new file license as suggested by @bz.

[dumbbell]

sys/compat/linuxkpi/common/src/linux_time.c

11 ↗

(On Diff #183793)

I renamed the file to suggest that it only contains timesub() and I fixed the license header. Does this match what you had in mind?

[bz]

I assume the copied file contents and logic are fine, so I haven't reviewed those.